1. Field of the Invention
The present invention relates to a technique for measuring a depth of a trench pattern formed on a substrate.
2. Description of the Background Art
A method of nondestructively measuring a depth of a trench pattern (for example, a set of a plurality of trenches extending in a direction) formed on a substrate by using a spectral interference method has been conventionally suggested. For example, Japanese Examined Patent Publication No. 6-65963 discloses a method of measuring a depth of a trench, where light is applied to a substrate having a trench, reflected light from the substrate is spectrally dispersed to obtain a spectrum, and then a cycle of peak in the spectrum caused by an optical path difference between the uppermost face of the substrate and the bottom face of the trench is specified by a maximum entropy method. Japanese Patent Gazette No. 3740079 also discloses a method of obtaining a depth of an etching trench. In the method, when the depth of the etching trench formed by etching a film on a substrate is measured, first, a spectrum which is obtained relatively to the film on the substrate with an original film thickness and a theoretical spectrum in a case where it is supposed that a thinner film than the original film thickness is formed on the substrate are acquired. The thinner film corresponds to the bottom face of the etching trench (that is to say, the depth of the trench is the difference between the original film thickness and a film thickness of the thinner film), and the spectrum of the film with the original film thickness and the spectrum of the thinner film are mixed with a mixture ratio in accordance with an area ratio of the etching trench in design, to acquire a plurality of mixture spectra corresponding to the etching trenches of a plurality of depths, respectively. The actual spectrum obtained from the substrate is compared with the plurality of mixture spectra to obtain the depth of the etching trench.
A spectroscope having a diffraction grating is frequently used in acquisition of a spectral reflectance. In the diffraction grating, a diffraction efficiency which is a ratio between an incident intensity and a reflected intensity of light is largely different between p-polarized light and s-polarized light depending on a wavelength of the light. In measurement of the depth of the trench pattern formed on the substrate, an oscillation direction of the reflected light from the substrate is limited by influence of the trench pattern in accordance with various conditions (that is to say, the reflected light from the substrate becomes polarized light). Thus, there are situations where the spectral reflectance cannot be accurately obtained on the basis of the reflected light from the substrate depending on the oscillation direction of the reflected light which enters the diffraction grating, and the depth of the trench pattern cannot be obtained with accuracy.
In measurement of the depth of the trench pattern on the substrate having a single layer film or a multilayer film, when a measured spectral reflectance acquired from the reflected light from the substrate and calculated spectral reflectances obtained by a computation are compared, the influence of the film in the calculated spectral reflectances needs to be considered because the measured spectral reflectance is affected by the film. However, in a case where a film on the substrate is extremely thin (for example, a film thickness of 10 nanometer (nm)), if it is tried to compare the measured spectral reflectance with the calculated spectral reflectances where a film thickness of the film is also included in parameters to obtain the film thickness of the film and the depth of the trench pattern, values of the parameters cannot be determined with accuracy. This is the same as in the case where the multilayer film is formed on the substrate.